This invention relates to a novel process for the production of photoconductive cadmium sulfide powder suitable for use in electrophotographic materials. More particularly, this invention relates to a process for the production of photoconductive cadmium sulfide powder of high quality by a one-step baking treatment wherein cadmium sulfide powder obtained by such a method as a precipitation method is used as raw material.
Photoconductive cadmium sulfide powder is widely used for a photosensitive material for a photosensitive element of electrophotography comprising a substrate with electrode function coated with a layer of such cadmium sulfide powder having a thickness of several ten microns by the aid of a binder such as a synthetic resin and overcoated with a top layer of a transparent high insulating material.
The photoconductive cadmium sulfide has hitherto been prepared by either the two-step baking method wherein cadmium sulfide incorporated with a flux and an activator is baked in the first baking step and the product is pulverized, washed with water and then baked in a sulfur-containing atmosphere in the second baking step or the three-step baking method wherein the first baking step is further divided into two steps. Because of a number of steps involved, however, these methods have many drawbacks including fluctuation in quality of the resulting photoconductive cadmium sulfide and reduction in yield of the product, thus resulting in increase of cost. In particular, the pulverization step carried out between the first and second baking steps or between two second and third baking steps creates a primary cause of the undue fluctuation in quality of the product, since such pulverization step serves also to deform the crystals once formed and fails to reproduce a narrow definite distribution range of particle sizes, thus exerting significant influence on photoconductivity. For obtaining photoconductive cadium sulfide of high sensitivity, the baking and other treatments should adequately be controlled so that the so-called dark resistance of cadium sulfide may fall within a suitable range which is not too high nor too low. However, the prior art methods consisting of a number of steps are not only difficult in control of conditions adopted therein but also abundant in uncontrollable factors such as the pulverization step referred to above. In the prior art methods, therefore, there has often been observed the case wherein the dark resistance is not balanced within a suitable range. In the prior art methods, it was necessary to add a relatively large amount, for example, 0.1-2% by weight of sulfur in the final baking step. The existence of such a large amount of sulfur apparently caused reduction in photosensitivity of the product.
The photosensitivity of such a photosensitive element as is described above has a relation with the particle diameter of the cadium sulfide powder as well as the distribution of the particle sizes of the powder, in addition to photoconductivity and dark resistance of cadium sulfide. As a particle diameter of about 2.mu. and a narrower distribution range were supposed to be preferable, control of such factors was made in the prior art methods by the treatments in the pulverization step and in the decantation step after washing the powder with water. In the prior art methods involving such pulverization and decantation steps, the particle size of the powder is distributed in a wide range and fine particles smaller than 0.5.mu. in diameter which are said to be undesirable are produced indeed in an amount of about 30%. However, these treatments are inherently difficult to control and thus can hardly be controlled entirely so that reduction in photosensitivity of the product and fluctuation in quality thereof are unavoidable.
In the above situation, there is a great demand for developing cadmium sulfide powder of high photoconductivity which overcomes the above metioned drawbacks and is devoid of any fluctuation in quality as well as a process for producing such excellect cadmium sulfide powder.